Liquid inks are widely used in a variety of printing processes, for example offset, intaglio, rotogravure, ink jet, electrographic and electrophotographic printing or proofing. Many of the desired characteristics of liquid inks are the same for each of the respective processes even though the final ink formulations may be substantially different. For example, in printing processes, it is desirable for the inks to remain in a free flowing liquid state during the ink deposition step, yet undergo rapid self-fixing shortly thereafter to produce durable, non-smearable, "prints" on a final receptor material. It is further understood that various rheological characteristics of the ink are known to affect its printing and transfer performance, e.g. ink "tack" and ink film modulus. The art continuously searches for novel methods to control and improve the rheological characteristics of liquid inks, particularly the rate of self-fixing, which in turn yields better print quality, efficiency and higher speed in the various printing processes.
In electrophotographic applications, which include devices such as photocopiers, laser printers, facsimile machines and the like, liquid inks are referred to as liquid toners or developers. Generally, the electrophotographic process includes the steps of forming a latent electrostatic image on a charged photoconductor by exposing the photoconductor to radiation in an imagewise pattern, developing the image by contacting the photoconductor with a liquid developer, and finally transferring the image to a receptor. The final transfer step may be performed either directly or indirectly through an intermediate transport member. The developed image is usually subjected to heat and/or pressure to permanently fuse the image to the receptor.
Liquid toners typically comprise an electrically insulating liquid which serves as a carrier for a dispersion of charged particles known as toner particles composed of a colorant and a polymeric binder. A charge control agent is often included as a component of the liquid developer to regulate the polarity and magnitude of the charge on the toner particles. Liquid toners can be categorized into two primary classes: for convenience, the two classes will be referred to as conventional liquid toners and organosol toners.
Of particular utility are the class of liquid toners which make use of self-stable organosols as polymeric binders to promote self-fixing of a developed latent image. U.S. Pat. Nos. 3,753,760; 3,900,412; 3,991,226; 4,476,210; 4,789,616; 4,728,983; 4,925,766; 4,946,753; 4,978,598 and 4,988,602 describe the composition and use of these types of organosols. Self-stable organosols are colloidal (0.1-1 micron diameter) particles of polymeric binder which are typically synthesized by nonaqueous dispersion polymerization in a low dielectric hydrocarbon solvent. These organosol particles are sterically-stabilized with respect to aggregation by the use of a physically-adsorbed or chemically-grafted soluble polymer. Details of the mechanism of such steric stabilization are provided in Napper, D. H., Polymeric Stabilization of Colloidal Dispersions, Academic Press, New York, N.Y., 1983. Procedures for effecting the synthesis of self-stable organosols are known to those skilled in the art and are described in Dispersion Polymerization in Organic Media, K. E. J. Barrett, ed., John Wiley: New York, N.Y., 1975.
The most commonly used non-aqueous dispersion polymerization method is a free radical polymerization carried out when one or more ethylenically-unsaturated (typically acrylic) monomers, soluble in a hydrocarbon medium, are polymerized in the presence of a preformed amphipathic polymer. The preformed amphipathic polymer, commonly referred to as the stabilizer, is comprised of two distinct repeat units, one essentially insoluble in the hydrocarbon medium, the other freely soluble. When the polymerization proceeds to a fractional conversion of monomer corresponding to a critical molecular weight, the solubility limit is exceeded and the polymer precipitates from solution, forming a core particle. The amphipathic polymer then either adsorbs onto or covalently bonds to the core, which core continues to grow as a discrete particle. The particles continue to grow until monomer is depleted; the attached amphipathic polymer "shell" acts to sterically-stabilize the growing core particles with respect to aggregation. The resulting core/shell polymer particles comprise a self-stable, nonaqueous colloidal dispersion (organosol) comprised of distinct spherical particles in the size (diameter) range 0.1-0.5 microns.
The resulting organosols can be subsequently converted to liquid toners by simple incorporation of the colorant (pigment) and charge director, followed by high shear homogenization, ball-milling, attritor milling, high energy bead (sand) milling or other means known in the art for effecting particle size reduction in a dispersion. The input of mechanical energy to the dispersion during milling breaks down aggregated pigment particles into primary particles (0.05-1.0 micron diameter) and "shreds" the organosol into fragments which adhere to the newly-created pigment surface, thereby acting to sterically-stabilize the pigment particles with respect to aggregation. A charge director may physically or chemically adsorb onto the pigment, the organosol or both. The result is a sterically-stabilized, charged, nonaqueous pigment dispersion in the size range 0.1-2.0 microns, with typical toner particle diameters between 0.1-0.5 microns. Such a sterically-stabilized dispersion is ideally suited for use in high resolution printing.
As with many printing inks, rapid self-fixing is a critical requirement for liquid toner performance to avoid printing defects (such as smearing or trailing-edge tailing) and incomplete transfer in high speed printing. A description of these types of defects and methods of preventing them using film forming compositions are described in U.S. Pat. Nos. 5,302,482; 5,061,583; and 4,925,766.
Another important consideration in formulating a liquid toner is the tack of the image on the final receptor. While toner tack is frequently an essential requirement for image transfer to the final receptor, it is desirable for the image on the final receptor material to be essentially tack free over a fairly wide range of temperatures. If the image has a residual tack, then the image may become embossed or picked off when placed in contact with another surface. This is especially a problem when printed sheets are placed in a stack. If the image is tacky, it may transfer to the backside of the adjacent sheet. To address this concern, a film laminate or protective layer is typically placed over the surface of the image. This laminate often acts to increase the effective dot gain of the image, thereby interfering with the color rendition of the color composite. In color proofing applications, a change in the color rendition makes it more difficult to ascertain whether the contract proof matches the printed sheet. In addition, lamination of a protective layer on top of the final image surface adds both extra cost of materials and extra process steps to the printing or proofing process.
Various means have also been used to address this problem by, for example, employing radiation or catalytic curing methods to cure or crosslink the liquid toner after the development step in order to eliminate tack. Such curing processes are generally too slow for use in high speed printing processes. In addition, such curing methods can add significantly to the expense of the printing process. The curable liquid toners frequently exhibit poor shelf stability and may result in brittleness of the printed ink.
Jordan, E. F., et al., Journal of Polymer Science, Part A-1,9, pp 1835-1852 (1971) (and references cited therein) report the heats of fusion and melting temperatures for selected (meth)acrylic homopolymers having n-alkyl monomer chain lengths between 12 and 22 carbon atoms which exhibit crystalline behavior. Jordan et al. note that polyacrylate homopolymers prepared using monomers with more than 13 carbon atoms in the main chain, and polymethacrylate homopolymers prepared using monomers having more than 17 carbon atoms in the main chain, exhibit crystalline melt transitions above room temperature (22.degree. C.).
The introduction of these types of crystallizable side chains into waterbased pressure sensitive adhesives has been shown to control the tackiness of the adhesive. For example, emulsion polymers with side chain crystallinity have been used to achieve temperature switchable tack. Clarke, R., et al., "Temperature Switchable Pressure Sensitive Adhesives", Adhesives Age, pp 39-41 (1993) describes the use of side chain crystallizable polymers to modify the melting point range of an acrylic emulsion polymer pressure sensitive adhesive. The side chains were shown to be capable of crystallizing independently of the backbone which provided a crystalline to amorphous transition that was reversible. This reversible transition proved to be useful for the development of pressure sensitive medical tapes to reduce skin trauma. U.S. Pat. No. 5,156,911 also describes the use of side-chain crystallizable and main-chain crystallizable polymers to produce temperature-sensitive pressure sensitive adhesives for medical applications. Both disclosures are limited to modifications of emulsion polymer pressure sensitive adhesives.
Yet another important consideration in formulating a liquid toner relates to the durability and archivability of the image on the final receptor. The resistance of the image on the final receptor to removal by abrasion, particularly by abrasion from natural or synthetic rubber erasers commonly used to remove extraneous pencil or pen markings, is a desirable characteristic of liquid toners.
Currently, no one has sufficiently addressed the problem of obtaining rapid self-fixing liquid toners for use in high speed color printing or proofing processes which do not exhibit one or more of the deficiencies described above.